The present invention relates to the field of semiconductor manufacturing and in particular to optical metrology of wafer thickness and high aspect ratio (HAR) structures etched into wafers.
Low coherence interferometry (LCI) or optical coherence tomography (OCT) is an optical measurement technique used to measure spacing between interfaces that are separated by optical distances much, much larger than >>λ, the wavelength of light used in the measurement. Typical set-up is shown in FIG. 1A. Light beam 101 from a broadband source 100 is split by a beam splitter 500 into a first portion 101A directed towards a sample 300 and into a second portion 101B directed towards the movable reference mirror 200. The reflected beams 101A′, 101A″ and 101B′ are recombined at the beam splitter 500 and directed towards photo diode detector 400.
In standard time domain low coherence interferometers/optical coherence tomography, the measurement arm 103 (the beam path that contains the sample 300) and the reference arm 102 (the beam path that contains the movable reference mirror 200) are initially adjusted so that the reflected beams 101A′, 101A″ and 101B′ interfering at the detector 400 in tandem have zero path-length difference (ZPD) providing a large intensity spike 4001 and 4002, as shown in FIG. 1B, and a corresponding large electrical output (not shown) from detector 400.
The first intensity spike 4001 appears when the reference beam 101B′ and the reflected beam 101A′ reflected from surface 301 have zero path-length difference. As the reference mirror 200 is moved, intensity spikes appear every time a zero path-length difference condition is met for a corresponding reflected light beams 101A′, 101A″ reflected from each optical interface 301, 302 and the reference beam 101B′. The distance D traveled by mirror 200 between adjacent intensity spikes 4001 and 4002, as shown in FIG. 1B, corresponds to the optical separation (optical thickness) between those two interfaces 301, 302.
Auto-correlation Low Coherence Interferometer . . . Time Domain: In many implementations of low coherence interferometers, the measurement beam from beam splitter 500 is directed to the sample 300 via an optical fiber. The same optical fiber bundle is also used for receiving the reflected beams 101A′, 101A″. Since the optical fiber bundle can be several centimeters to meters long, temperature fluctuations in the measurement path will significantly influence the thickness measurements. To minimize this error, an auto-correlation approach is implemented as described in U.S. Pat. No. 7,426,036 and US Patent Application 20090065478.
In auto-correlation low coherence interferometers, the reflected beams 101A′, 101A″ reflected from the sample 300 enter an auto-correlator optical unit 1000 as shown in FIG. 2A. Reflected beams 101A′, 101A″, 101B′ from the different interfaces 301, 302 and the reference mirror 200, respectively, are amplitude divided and then recombined by the beam splitter 600 in the auto-correlator optical unit 1000. When the mirrors 201 and 202 are equidistant from the beamsplitter 600, a zero path-length difference condition is met for all interfaces (that is return from 200 and 301 and 302 here) and the detector 400 will produce an intensity spike 2001. This condition is schematically shown in FIG. 2B. When mirror 201 is moved away from beamsplitter 600, the detector 400 signal will drop sharply as the overlapping beams move away from the zero path-length difference condition. As the mirror 201 is further displaced from the beamsplitter 600, an increase in detector 400 output which corresponds to intensity spike 2002 will be observed at a particular location of mirror 201. This signal strength corresponding to intensity spike 2002, though smaller than the first signal strength corresponding to intensity spike 2001, represents a zero path-length difference condition between reflected beam 701A′ reflected from mirror 201 in arm 701 and the reflected beam 702A′ reflected from interface 301 in arm 702. This situation is represented schematically in FIG. 2C. E-field 201′ refers to reference beam 701A reflected from mirror 201 and e-field 202″ refers to reference beams 702A′ reflected from mirror 202. Similarly E-fields 301′ and 301″ refer to beams 101A′ reflected from interface 301 and e-fields 301′ and 302′ refer to beams 101A″ reflected from interface 302. E-fields 301′ and 302′ refer to beams from arm 701 and e-fields 301″ and 302″ refer to beams from arm 702. Referring to embodiment in FIG. 2A, those skilled in the art will notice that the location of reference mirror 200 can be in the beam path 103.